Speed-indicator for electric motors.



PATENTED'J'ULY 21, 1903.

E- ,J. BERG. SPEED INDICATOR FOR ELEGTRIG 'MOTORS.

APPLICATION FILED MAR. 27, 1902.

N0 MODEL.

INVELISITOD EJHSC LL56? M UNITED STAT S A} Patented July 21, 1 903.

PATENT OFFICE.

ERNST J. BERG, OF SOHENEOTADY, YORK, ASSIGNOR TO GENERAL ELECTRICCOMPANY, A CORPORATION OF NEW YORK.

SPEED-INDICATOR FOR ELECTRIC MOTORSL SPECIFICATION forming part ofLetters Patent No. 734,455, dated July 21, 1903.

Original application filed August 24, 1898, Serial No. 689,373. Dividedand this application filed March 27, 1902. Serial No. 100,194. (Nomodel.)

To all whom it may concern:

Be it known that I, ERNST J BERG, a citizen of the United States,residing at Schenectady, county of Schenectady, State of New York,

have invented certain new and useful Improvements in Speed-Indicatorsfor Electric Motors, (a division of my prior application, Serial No.689,373, filed August 24, 1898,) of which the following is aspecification.

In the operation of induction-motors it is well known that the speeddrops as the load increases. This drop in speed may be considerable insome instances without being seriously detrimental. In some cases,however,

i 5 a drop in speed as great as two or three per cent. is notpermissible, and this is particularly true in cases whereinduction-motors are used for running cotton or other textile machinerywhere even a slight speed variation may have a very destructiveinfluence. In order to fulfil the requirement of close speed regulationthus called for, it is necessary to make the motors very large, in whichcase they become very expensive in propor- 2 5 tion to the amount ofpower developed. In-

stead of providing large motors with close regulation the same objectmay be accomplished by using smaller'or less expensive motors andpreventing too great a drop in 0 speed by correspondingly speeding upthe generators in the power-station. To do this, it is necessary tohavesome method forindieating either the speed or drop in speed of the motorin the receiving-station.

35. The uses to which such a method may be put are many and various, sothat for purposes of illustration I have described only one relation inwhich my invention is of value.

In the drawings, Figure 1 is a diagrammatic representation of myinvention, and Fig. 2 is a view of one form of apparatus which I employ.

In carrying out my invention for indicat- 5 ing at a distance the speedof an inductionmotor I make use'of the fact that currents are developedin the inducedmember of the motor of a frequency corresponding to theslip, or, in other words, to the diiference be- 5o tween the actualspeed of the motor and that which it would have at absolutesynchronisin. By tapping the windings of the induced member at suitableintervals currents may be taken off of a frequency corresponding to theslip. If these currents be conducted to any desired point andthe'frequency of the currents there measured, a result is obtainedindicative of the speed of the'induction-motor.

In the drawings, I represents the induction-motor the speed of which isto be meas- 6o ured. The induced member 1 of the motor is tapped atpoints separated from each other by an angle equal to one-third of thepolar pitch. Three-phase currents are taken ofi through collector rings2 and brushes 3. Owing to the low frequency atwhich these currents aregenerated, their voltage may be too low to economically run thefrequencyindicator F. To remedy this objection, a step-up transformer Tof any suitable char- 7o acter may be employed. The secondary mains 4lead to the frequency-indicator F, which, as before stated, may belocated at any desired point.

The frequency-indicator may assume many forms without departing from thespirit of my invention; but the form which I prefer'to employ and whichI have illustrated in the drawings consists of a motor of therotary-field type, in which one member, 5, isfed with polyphase currentof the same frequency as that fed to the motor whose speed is to bemeasured, while the other member, 6,-is fed with current from thestep-up transformer T, the latter current, as before stated, having afrequency dependent upon the slip of the motor.

It is within my invention, however, to con struct thefrequency-indicator in the form of a synchronous motor in whichthe fieldis fed by direct current from any suitable source. 0 In this case thespeed of the frequency-indicator corresponds to the slip of theinductionmotor.

Inthe normal operation of the preferred form of frequency-indicator F,inwhich both 5 members are supplied with energy from the same mains, thepolar lines producedin its two relatively rotatable members willcoincide, and the speed ofthe revolving member will be such as toaccomplish the result. Thus 10o it the polar lines have the samedirection of rotation in space then the revolving member will have aspeed less than that of synchronism by an amount equal to the slip ofthe motor I. In other words, the speed of the frequency-indicator F isthe same as that of the motor whose speed is to be measured, so that allthat is necessary in order to determine the speed of the motor I is tomeasure the speed of rotation of the revolving member of thefrequency-indicator F. This may be done by a tachometer 7 of anysuitable character or by any other known means for measuring the rate ofrotation of a shaft. The reading of the tachometer gives the speed ofthe motor I. If the number of poles on the frequency-indicator F and themotor I differ, this reading will require to be increased or decreasedaccordingly; but in any case it will be proportional to the speed to bemeasured.

If the polar lines of the two relatively rotatable elements of thefrequency-indicator F revolve in opposite directions, then it is evidentthat the revolving member will rotate at a rate equal to the sum of therates of rotation of the polar lines-that is, at a speed equal tothespeed of synchronism plus that corresponding to the slip of the motorI. The reading of the tachometer minus a constant corresponding to thespeed of synchronism gives the slip of the motor I. Either method ofspeed-measurement may be employed as convenience may dictate.

If it be desired to measure the variation in speed of the machine fromthe speed corresponding to synchronism, itis necessary to obtain tworeadings from the frequency-indicator, one with the two polar linesrevolving in the same direction, the other with the two polar linesrevolving in the opposite direction. One reading represents the speed ofr synchronism minus the slip of the machine whose speed is to bemeasured, while the other reading represents the speed of synchronismplus the slip. One-half of the difference between these readingsobviously represents the variation in speed from synchronism of themachinewhosespeedisbeing measured. The mean of the two readings gives,of course, the speed of synchronism. In order to reverse the directionof rotation of the polar lines in either of the two members of thefrequency-indicator in order to obtain these two readings, it is onlynecessary to transpose the connections of two of the leads supplyingcurrent to that member. Although I have described two methods ofmeasuring the speed of an induction-motor, one of which gives directlyeither the exact speed of the rotating member or a multiple thereof,while the other gives a result corresponding to the speed of synchronismplus a quantity equal to the slip or drop in speed of. the motor, Iprefer ordinarily to employ that method of speedmeasurement which givesdirectly the speed of the motor rather than resort to a method whichpresupposes a knowledge of the speed of synchronism, since this speedcannot be definitely known unless specially measured. It is obvious,nevertheless, that the methods of speed-measurement described by meother than that which I prefer are equally within my invention.

It will be evident to those skilled in the art that my invention is notlimited in its application to the measurement of the speed ofinduction-motors, but may be employed to measure the speed of any devicein which alternating currents are developed of a frequency dependentupon the speed. Thus, for example, the armature of a direct-currentmotor may be tapped at suitable intervals and the resulting alternatingcurrent fed to one member of a synchronous motor the field of which isexcited by direct current. The speed of the synchronous motor willcorrespond to the speed of the armature of the diroot-current machine.In the same manner a synchronous motor might, as before stated, he usedin place of that form of frequency-indicator illustrated in thedrawings. Under such circumstances the synchronous motor would indicatesimply the slip of the induction-motor rather than its actual speed.Many other modifications will suggest themselves without departing fromthe spirit of my invention.

What I claim as new, and desire to secure by Letters Patent of theUnited States, is-

1. The method of measuring the slip of an induction-motor, whichconsists in developing in the induced member of the motor current of afrequency proportional to the slip, supplying said current to a devicehaving a speed of' rotation proportional to the frequency of saidcurrent, and measuring the speed of said device.

2. The method of measuring the variation from synchronous speed of arotary-field dynamo-electric machine, which consists in developing inone member of the machine an alternating current the frequency of whichis proportional to said variation, transmitting said current to a deviceresponsive thereto, and observing the indications of said device.

3. The method which consists in supplying polyphase current to onemember of a dynamo-electric machine of the rotary-field type, developingin the other member of the machine a current of a frequency dependent onthe variation in speed of said machine from synchronism, raising thepotential of the current thus developed, transmitting the same, andsupplying it to one member of a motor, supplying current to the othermember of the motor and measuring the speed of its revolving member.

4. The method which consists in supplying polyphase current to onemember of a dynamo-electric machine of the rotary-field type,

chine a current of a frequency dependent on the variation in speed ofsaid machine from In Witness whereof I have hereunto set my synchronism,transmitting said current and hand this 26th day of March, 1902. Isupplyin it to one member of a frequencymeasuring, device, the othermember of which I ERNST BERG is supplied with current of the samefrequency as that supplied to said dynamo-electric machine. I

Witnesses:

BENJAMIN B. HULL, HELEN ORFORD.

